An abnormal mass of tissue. Tumors are a classic sign of inflammation, and can be benign or malignant (cancerous). There are dozens of different types of tumors. Their names usually reflect the kind of tissue they arise in, and may also tell you something about their shape or how they grow. For example, a medulloblastoma is a tumor that arises from embryonic cells (a blastoma) in the inner part of the brain (the medulla). Diagnosis depends on the type and location of the tumor. Tumor marker tests and imaging may be used; some tumors can be seen (for example, very advanced tumors of the breast) or felt (palpated with the hands). Cancer may be found when it is very small, and in such cases, cannot be felt. Although there is no “tumor” this would still be a cancer, if the cells show the typical cancer characteristics as visualized under the microscope.
The use of the word tumor is sometimes confusing, since it doesn’t always mean cancer.

A gene that protects a cell from one step on the path to cancer. When this gene is mutated to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes.